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By Krishna V. Jog
Technical Advisor
Kirloskar Pneumatic Co. Ltd., Pune
Krishna V. Jog is a first batch IIT Bombay graduate in mechanical engineering and also has an M.Tech from the same institute. He has a total of 42 years experience of which 8 years are in teaching and research and 34 years in the ACR industry. He retired as MD of Kirloskar McQuay. He is a presidential member of ISHRAE, Pune chapter as well as an ASHRAE member. Can be reached at kvjog@vsnl.net
The world of refrigeration was born with the use of reciprocating compressors. This was the only type of compressor available in the earliest days of HVACR for any application.
The word 'open' means which is open to atmosphere or surrounding ambient. There is no such compressor, which is really open. To that extent, it is a misnomer to call it an 'open' compressor. ASHRAE defines an open-type compressor as a refrigerant compressor with a shaft or other moving part extending through its casing to be driven by an outside source of power, thus, requiring a shaft seal or equivalent rubbing contact between a fixed and a moving part.
A reciprocating compressor is a positive displacement compressor that changes the internal volume of the compression chambers by the reciprocating (to and fro) motion of one or more pistons. Recips are single acting, using trunk type pistons, driven directly through a pin and connecting rod from the crankshaft. In the very old days, double acting recips were used which had piston rods, cross heads, stuffing boxes and oil injection. These looked very similar to huge steam engines and used to run at very low speeds. They are obsolete now, since they were very heavy, occupying large space and large flywheels.
Modern open recips have 1 to 16 cylinders in various designs such as, inline,V-shape and W-shape. The capacities vary from a fractional ton to as high as 400 ton in a single machine. The speeds can vary from about 250 rpm to 3,600 rpm or more. They are available for various refrigerants like halocarbons, CFC's, and ammonia. The compressors for ammonia application cannot use copper, brass or bronze materials in construction. They can only use steel / SS or aluminiumAmmonia reacts with non-ferrous materials. Recips are used in single stage or two-stage versions. The single stage units are generally designed for compression ratios of about 8 to 9.5. Some heavy-duty industrial units are designed compression ratios of upto 12, but they suffer from lower volumetric efficiencies.
The single stage machines used for ammonia can go down to about –10°C to –15°C evaporating temperatures with +40°C condensing temperature. The same compressor can go down to about –25°C to –30°C evaporating temperatures with +40°C condensing for typical R-22 or equivalent HFC refrigerants.
For temperatures lower than these, say in the range of –40°C and below, two-stage internally compounded compressors are available. They can go down to –60°C evaporating with +40°C condensing temperature. These are normally used for water-cooled condenser applications. For air-cooled applications, there are other types of open recips which can withstand high condensing temperatures of +60°C or above but these are restricted to evaporating temperatures of no lower than –5°C. For transport applications of refrigeration and/or air conditioning, lightweight, high speed relatively lower capacity machines are available.
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The other types of compressors are hermetic and semihermetic or accessible hermetic. Many a time, the word 'hermetic' is loosely used to include ‘semi-hermetics’ as well. In case of hermetic and semi-hermetics, the compressor and the motor are enclosed in one shell either by welding or by bolting. There is no shaft seal between the compressor and the motor shaft. In the olden days, shaft seals were not well developed and hence not reliable, tight and leak-proof. Hermetic compressor manufacturers took advantage to highlight the seal leakage. Over the years, dramatic improvements have taken place and excellent, leak proof, very reliable seals are now available. Hermetics were introduced keeping low first cost in mind – open machines demand a premium.
Every system has some merits and demerits, and it depends upon the boundary operating conditions to find out which type is best for a particular type of application. For ammonia and for many critical applications of industrial nature, open compressors are the only natural choice. Open systems have a tremendous number of advantages over hermetic / semi-hermetic systems. Some of the main factors where open systems score over hermetics are shown in Table 1.
| Table 1: Comparison between Open and Semi-hermetic / Hermetic Systems | |
|---|---|
| Open | Semi-hermetic / Hermetic |
| Open systems have motor separately connected to compressor. Not connected with refrigerant and oil charge in the system. | Any problem in motor affects refrigerant charge and oil charge. |
| Compressor being open design, can be inspected / opened by just closing the isolation valves. No botheration of removal / loss of refrigerant gas. | Motor and compressor are in the same casing. Maintenance at site in case of hermetic is impossible and very difficult for semihermetic machines. Some of the internationally well known brands recommend opening of semi-hermetic also only at the factory / service centre and not at site. |
| Since these can be attended to at site, down time is very minimal. | If the compressor has to be taken to factory, long unavoidable delays will result and disrupt complete cooling. |
| Since motors are outside the refrigerant environment, wide choice of makes and type possible. Indian made motors are easily available and can be replaced without complete shut down of the plant. | Hermetic / semi-hermetic motors are imported. These are generally not manufactured in India and not readily available for replacement. If motors are rewound in India, the quality of windings is doubtful and may not be as good as original. (This comment applies only to imported compressors). |
| Motor windings are cooled by ambient air. | Motor windings are in the refrigerant vapour surroundings. |
| Open machines are costlier compared to hermetic. | Hermetic machines were basically introduced to reduce manufacturer's first cost. Motor is smaller and cheaper as it is cooled by the refrigerant. Copper and iron content are about half that of an open design. |
| Open motors do not require any refrigeration effect and thus do not reduce the capacity of the system | Hermetic motors consume cooling energy produced by the refrigerant. These vapours are to be compressed by the same compressor, hence, effectively some cooling capacity is lost in motor cooling itself. |
| Open system is about 10% more efficient considering loss in cooling motor and penalty in power for compressing vapour. | Net effectiveness of cooling capacity and power consumption is about 10% poorer due to hermetic / semi-hermetic design. |
| Open motors are more efficient as they are cooled / ventilated by ambient air. | Hermetic motors are less efficient as they are cooled by refrigerant and are loaded beyond peak efficiency point. These motors are rotating in a much denser refrigerant atmosphere and have higher windage loss. |
| Power fluctuations are not highly detrimental to open motors. | Power fluctuations and electrical transients can produce a flash in the refrigerant atmosphere, which can break down into carbon, fluorine, chlorine etc. and will be carried into the system. When combined with moisture, hydrochloric and hydrofluoric acids are formed which can cause large – scale contamination. |
| Insurance premia charged for open systems is much less as the damages are not catastrophic and not for long. | Hermetics will take very long before they are put back into operation, hence the insurance companies charge much higher premium as the refrigerant and oil invariably need to be replaced. |
| Overall maintenance is very fast, quick and without any problem on refrigerant side. | Heavy maintenance in case of burnouts such as cleaning, flushing, vacuumising, pressure testing and recharging fresh refrigerant and oil. |
| The refrigerant, if ammonia used, is totally safe, environmentally acceptable and free from Montreal Protocol or phase-out problems. The refrigerant NH3 (R 717) has zero ODP and zero GWP. It has a low TEWI factor. If the refrigerant leaks into atmosphere, it returns back to soil as fertilizer, since it mixes with water in any proportion. | The refrigerant used is either HFC or HCFC, having a lot of limitations of ODP, GWP, phase-out and TEWI values. If HCFC is used, fast phase-out is imminent. If HFC is used, high GWP values make them unacceptable even in Europe and many other countries. |
| The refrigerant NH3 is very cheap (economical) and manufactured locally in India in plenty and available freely. | HFC refrigerants used are not manufactured in India. They are very costly and cannot return to soil if leaked. ODP and/or GWP problems are plenty. |
| Oil being heavier than refrigerant, can be drained easily even during running. Oil available and manufactured locally in India. | Oil is partially miscible and oil recovery and maintaining the oil level is a big problem. Invariably oil has to be imported and very costly. |
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This is identified in three different ways: 1) EER 2) COP 3) kW / TR or bhp / TR. For open recips, it is the bhp / TR that is most commonly used. For open compressors, there is nothing like "input kW" as there are no electrical terminals fitted to the compressor motor, as is the case with hermetic or semi-hermetic compressors. In case of hermetics, (which will also include semihermetics throughout this paper) there is no way to measure brake horse power or brake kW since the compressor shaft is inside the common shell for compressor and motor. The motor efficiency thus comes into the picture for such hermetics. If high efficiency motors (97% to 98%) are used, a lower ikW figure would be obtained and higher ikW would result in lower efficiencies. Typically, for small compressors and small motors, motor efficiency can be as low as 60% or so. For open compressors, brake horse power is measurable and, therefore, for open systems only bhp / TR can be defined.
The efficiency or effectiveness of a refrigeration system depends on several factors. The ratings published must give the datum or the basis for such data. Many-a- times, these factors are not given and then a true and fair comparison of various systems cannot be made. Some of the main factors on which the capacity and power consumption depend are: 1) Speed, 2) Degree of sub-cooling, 3) Degree of superheating, 4) Pressure drops allowed in suction and discharge. Let us consider these briefly:
ARI (Air conditioning & Refrigeration Institute, USA) has developed a rating system and given basic parameters at which these can be compared. Most of the time, these ARI set of parameters are non-existent in India, but it is often used as a common basis for comparison. Parameters like cooling capacity TR, power consumption, kW or bhp, COP etc. can be compared, if they are brought on the same basis and if a comparison is made, “apple to apple”. In actual practice, the plant never works at design conditions and the performance has to be judged at off-peak or off-design conditions. ARI provides this by comparing the Integrated Part Load Value (IPLV) for various offers. The IPLV uses weighted national weather averages, weighted averages of building operation and a blending of the kW/TR at four rating points: 100%, 75%, 50% and 25%.
Screw compressors are excellent rotary machines, which are also positive displacement machines. They are of two designs – mono screw or dual screw. The screws are basically capable of high compression ratios in a single stage even upto 20 to 25. They can also be used for multistage operations. Screws are fully balanced rotary machines with high precision required for manufacturing of screws. A large quantity of lubricating oil is used and they require a special oil tank separator.
The actual screw compressor is a very compact of equipment, but with the motor, control panel, (Oil Tank Separator) etc. it looks very bulky.
From a typical screw compressor performance table with economiser, it can be seen that at a moderate temperature of 35°F evaporating and 115°F condensing, it consumes 1.2bhp / TR whereas, a typical open recip compressor consumes less than 1 bhp / TR. These figures are for R-22 refrigerant and for Ammonia the recips consume even lower power than that of R-22. Thus, in many instances, the power consumption of screws at full load is higher than that of recips. The difference in power consumption at lower temperatures of around – 3 0 ° C / – 3 5 ° C evaporating with +40°C condensing and for Ammonia is much more and recips become clearly the winner. Screws consume much more power, apart from oil management system complications.
The same story is true for part load operation as well. A typical IPLV comparison at ARI conditions is shown in Figure 1 for screws and recip chillers6.
Another comparison of part load performance of piston (recip) compressor and screw compressor is shown in Figure 25.
Also, at high ambient conditions with typical heat pump applications the design EER for recips, is much better than screws as shown in Figure 36.
There is a general belief that screws are better than recips but this is a myth as shown, both at full load as well as part load operations. Recips cost much less than screws and are real work-horses if proper, minimum preventive maintenance is carried out.
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The most common refrigerant for open recips for industrial applications is, without any iota of doubt, inexpensive Ammonia. It has very high latent heat and low specific volume, hence gives the maximum capacity per cfm (m3/hr) of compressor swept volume. To give you an idea, it is about 20% more than R-22 and 100% more than R-134a. Ammonia has zero ‘ODP’ (Ozone Depletion Potential) and zero ‘GWP’ (Global Warming Potential). It has no environmental problems. If leaked, it returns back to soil as fertiliser since it mixes with water in air / rain in any proportion. It is very economical since it costs very little in comparison to R-22 or R-134a. It is manufactured in India and available in plenty.
It cannot be readily used if the charge in the system is large, in public or residential areas because of its odour and toxicity. However, Ammonia is now becoming more popular with open recips because of the impending ban on R-22, an HCFC, as per Montreal Protocol and the dependence on imports of R-134a, together with a drastic reduction in capacity, use of synthetic oils (once again, imported) and global warming potential.
With the emergence of tighter systems, better good quality cap steel valves, negligible or very emissions and leakages, very good alarm monitoring detection systems, Ammonia is the obvious choice all industrial applications.
Recip compressors have been around since the inception of this industry. They were introduced even before the power costs zoomed around 1974 and energy became very costly with the formation of OPEC. The published performance tables and charts for various refrigerants for various types of recip compressors have been in use for over 100 years and there has been no manipulation in this rating information.
Screws and centrifugal compressors are relatively recent entrants and so are many new refrigerants with which these compressors are used. Keeping in mind high energy costs, some manufacturers try to claim lower kW/TR figures. These figures are based on various unrealistic assumptions and implicit parameters not known to many. Almost no installations or systems using recips have been found to give less capacity or consume more power than the published ratings as they were conservative figures and there was no reason to unnecessarily hike the performance. For later entrants with so many variations in assumptions and datums, it has been proved, even in the international market that many systems have failed to achieve the guaranteed figures even with the ARI tolerance.
Recip compressors are often blamed for poor performance and quality simply because they break down due to poor system design, wrong installation and bad commissioning practises.
All recip compressors are supplied with a detailed manual, which explains various guidelines for system design as well as the precautions to be taken during installation, start-up and commissioning. Sometimes, the P&ID are not correct and if they are correct, the actual installation is faulty and not in line with the P&ID. Many compressor failures take place because, proper care in cleanliness inside the refrigeration piping is not taken. Compressors can breakdown, fail and serious accidents can take place, if even simple precautions and recommendations are not strictly followed. Some examples are enumerated below:
System design. No sight glass in the liquid line, no strainer in the suction, no suction to liquid heat exchanger etc.
These are all very simple accessories, but very important for healthy working of a compressor. If a sight glass is not correctly installed at the right place or not installed at all, we will never know if full liquid is going to the evaporator or not. Flash gas to the evaporator will yield lower cooling capacity and higher kW/TR and poorer COP.
Strainer in the suction line removes the dirt in the system. The inside bags must be replaced as per the recommendations or serious pressure drops in suction and heavy wire drawing will occur.
Inclusion of suction and liquid heat exchanger ensures superheated vapours to the compressor without liquid slop-overs. Many systems are installed without such a heat exchanger.
Wrong size suction / delivery stop valves on compressor. Using smaller sizes than recommended sizes will lead to breakage of pistons, liners, connecting rods and valves due to high velocities, high pressure drops and wire-drawing effects.
System piping not thoroughly cleaned. All dirt, welding slag etc. can enter the filter and compressor and cause pump failure, lube oil failure, pump shaft breakages and then bearing failure.
Incorrect oil for lubrication. Recommended oil grades are of refrigeration quality. Sometimes, nonrefrigeration oils having same reference number only, are used which destroys the complete compressor. Our recommendations are Hindustan Petroleum Seetul -68, Indian Oil Servofriz-68, Bharat Petroleum Bharat Freezol-68. Instead of these, use of Servo system brand having same viscosity number, to be used for air compressors and not for refrigeration compressors can give rise to complete damage of the compressor.
Incorrect operation of two-stage internally compounded compressors. All two-stage compressors must be operated with caution. They should operate only as single stage (HP) cylinders till the pull-down conditions from high ambient are achieved. Until the evaporating temperature comes down in the vicinity of –15 °C to –20°C, it should not be operated as a two-stage compressor. It is also necessary to check that the system is designed for pull-down conditions with both the motor kW and the condenser being capable of taking the load at the suitably low part-load operation. If not operated properly, the intermediate pressure and temperature can rise abnormally high and damage big-end bearings of high-pressure connecting rod and also crank pin and even a crankshaft.
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After commissioning of the system, whether it is for chilled water or for air conditioning with DX coil, whether the system is water-cooled or air-cooled, proper log data must be maintained. The readings must be taken, initially at an interval of 1 or 2 hours and later on as the system stabilises, the readings must be taken at least once every 3 or 4 hours. The log data readings will be useful for analysing faults and corrective actions. A typical log sheet for reciprocating compressors with chiller or DX coil for air-cooled air conditioning plants is shown in Table 2. Proper variations in this format can be made for a particular type of system if it is different than the one given in the log sheet.
| Table 2: Recommended Log Format for Air-cooled Air Conditioning plant Chiller or DX coil | ||
|---|---|---|
| Date of Log | Compressor Model | Refrigerant |
| 1 | Compressor run hours (Time ) | Hrs. |
| 2 | Unit capacity (Loading) | % |
| 3 | Entering evaporator liquid temperature (CHW in) | °C |
| 4 | Leaving evaporator liquid temperature (CHW out) | °C |
| 5 | Difference of Sr 3 & 4 ( Δt ) | °C |
| 6 | Evaporator water flow | LPM |
| 7 | Cooling Capacity, (Sr.6/60) x 4.186 x Sr.5) | kW |
| 8 | Cooling Capacity, ( Sr.7/3.516) | TR |
| 9 | Saturated suction pressure | kg/cm2g |
| 10 | Saturated suction temperature | °C |
| 11 | Suction line temperature | °C |
| 12 | Suction superheat, (difference of Sr.11 &10) | °C |
| 13 | Ambient temperature | °C |
| 14 | Average face velocity of air over the coil (air-cooled units) | m /s |
| 15 | Number of fans ON at the time of log (air-cooled units) | # |
| 16 | Condenser air cfm | m3/hr |
| 17 | Saturated discharge pressure | kg /cm2g |
| 18 | Saturated discharge temperature | °C |
| 19 | Discharge line temperature | °C |
| 20 | Liquid refrigerant temperature leaving condenser | °C |
| 21 | Liquid sub-cooling (difference of Sr. 18 & 20) | °C |
| 22 | Diff. Oil pressure | kg/cm2 |
| 23 | Current in phases R/Y/B | Amps |
| 24 | Average current | Amps |
| 25 | Supply voltage | V |
| 26 | Power \/¯3 VI cosø | kW |
| 27 | Air Flow across cooling coil cfm | m3/hr |
| 28 | Return Air (entering to DX coil) | Dbt Wbt Enthalpy |
| 29 | Supply Air (from the DX coil) | Dbt Wbt Enthalpy |
| 30 | Cooling capacity of coil 4.5 x cfm x ( h28-h29 ) | TR |
| 31 | Fan motor amps (AHU Fan) | Amps |
| 32 | COP (TR / Power input) in same units | – |
| 33 | Sight glass status (clear /bubbles) | – |
Notes :
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There are some groups of people who advocate only screws. They go to the extent of saying that, recips are fast going out of the scene and they will soon be forgotten and dead. The answer to these is an emphatic No. We have seen in this article how ammonia is increasingly popular for all applications and how open recips have high COPs or EERs and how recips can automatically adjust to different discharge pressures, depending on different ambients not only in different seasons, but also during the day cycle itself. They are also lower in power consumption on full load and part load conditions.
For many applications, recip compressors will remain the compressor of choice.
With the cooling load determined one has a fix on the plant capacity. The plant capacity can be met by various types of systems:
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